The present invention pertains to thermal processing a semiconductor wafer. More particularly, the present invention relates to a method and apparatus for emissivity independent heating of the wafer during the processing.
Apparatus to fabricate a thin film using such techniques as chemical vapor deposition (CVD) or atomic layer deposition (ALD) are routinely used in the manufacture of semiconductor wafers. In a film forming apparatus, a reproducible temperature distribution across the wafer is extremely important for film uniformity.
The most common epitaxial (epi) film deposition reactors used in modern silicon (Si) technology are similar in design. A quartz reaction chamber contains the wafer support, the susceptor, which is rotated to improve the deposition uniformity across the wafer. Only one wafer is processed at a time. Process and carrier gases flow over the wafer in a laminar mode and parallel to the wafer surface. The wafer is heated by tungsten-halogen lamps located underneath and above the reaction chamber, radiating through the quartz and directly heating the wafer and susceptor. The lamps and the quartz walls of the chamber are air-cooled to protect the lamps and to prevent the risk of Si depositing on the reactor walls. The wafer is loaded and unloaded fully automatically and the reaction chamber is separated from the ambient by load locks and a wafer transfer chamber.
The susceptor can be a graphite disc with a SiC coating to smooth out local temperature variations from the radiant heat source. At temperatures in the 500-900xc2x0 C. range, the growth of epi Si (silicon) and SiGe (silicon germanium) is very temperature sensitive and epi growth is often effected on patterned wafers in selective or in blanket growth mode.
A patterned wafer (having circuitry and devices) can have a different emissivity characteristic than a blanket wafer. In addition, a pattern on a wafer can have a different emissivity characteristic than a different pattern on another wafer. Heat can be emitted from the wafer with the first pattern different than with the different pattern and the temperature distribution of wafers with different patterns can vary which can vary the deposition rate and characteristics of films being placed on the wafer. Since a varying thermal profile on a wafer can affect reaction rates, the varying thermal profile can determine a film deposition rate. As a result, a change in wafer patterns can require a re-tuning of the process to confirm that correct heating of the wafer is accomplished. In addition to film deposition processes, any process involving thermal treatment such as bake, anneal, etc. can be affected by the wafer emissivity.
Often in thermal processing reactors, such as in Epi CVD apparatuses, the substrate is heated from both the device side and the non-device side. Using this dual side heating approach, the temperature distribution across the wafer is very sensitive to the emissivity of the surface onto which a film is deposited. As a result, the deposition rate will be different at different locations on the wafer. The deposition rate also varies between wafers having different patterns on their front surface due to the different emissivity of these patterns. Moreover, the deposition rate can change during the deposition itself because the species which are being deposited can change the wafer""s emissivity. In addition to the dependence of the deposition rate on emissivity, the chemical composition of the deposited film will also be emissivity sensitive because incorporation of species into grown films can be temperature dependent.
An apparatus for heating and monitoring a wafer that reduces dependence of the temperature distribution along the wafer, to wafer emissivity, is disclosed. The apparatus provides a susceptor capable of holding a wafer that is placed in a process chamber within quartz domes. An array of lamps placed outside the quartz domes heat a susceptor backside. A reflector is placed outside the quartz domes to have a mirrored surface face the wafer device side and reflect heat back onto the wafer. The shape of the reflector is optimized to provide the best temperature uniformity. The chamber is designed to restrict light from the lamps from leaking around the susceptor to heat the wafer directly. An optical thermometer may be placed above the reflector to read a wafer device side temperature through a hole in the reflector.